As is well-known in the television art, the raster or deflection pattern produced on a spherical face of a cathode ray tube (CRT) suffers pin-cushion distortion as a function of the center of curvature of the screen. With non-spherically faced CRT's, the effect of pin-cushion distortion is more pronounced.
With the ever-increasing use of CRT's to display alpha-numeric and precise graphical data, rather than video or picture information, the constraints imposed upon the degree of pin-cushion distortion, miss-convergence and other forms of distortion have become much more rigid. In monitor applications, for example, it is imperative that the edges of the raster be capable of close control (e.g., straightness, size and linearity) to assure a display field having correct dimensions and straight sides.
It has been found that, during the manufacture of monitors, various factors result in imbalanced or differential E-W pin-cushion distortion. These factors include cocked yokes for yamming (i.e., to account for gun misalignment) and yoke winding manufacturing discrepancies. Thus, for example, E-W pin-cushion distortion may result with one edge of the video being distorted more than the other.
I have addressed the problem of distortions on the top and bottom of a CRT in my U.S. Pat. Nos. 4,845,409; 4,766,354; and 4,748,384 (all of which are assigned to the Zenith Electronics Corporation of Glenview, Ill.). Others have addressed the problem of E-W pin-cushion distortion: U.S. Pat. Nos. 4,794,307 (Raster Distortion Correction for a Deflection Circuit); 4,827,194 (Raster Size Regulating Circuit); and 4,206,388 (Current Control Circuit for Horizontal Deflection Coil of a Television Receiver). For the most part, the prior art has addressed the problem by concentrating on the raster portion of the video display or by modulating the horizontal deflection current flowing through the horizontal deflection coils. Unfortunately, those circuits provide the same amount of correction on both sides of the display so that, if there is a differential, some distortion will remain on one of the sides of the display after the correction is applied. Moreover, correction circuits which have been found to be satisfactory for use in a television display are not necessarily satisfactory when used in connection with a monitor. Thus, an adequate solution has yet to be found to the problem of E-W pin-cushion distortion imbalance, particularly in the context of a video monitor.